Translated from Russian. Among all practically useable metallic materials most consist of two-phase and multiple-phase alloys. To study the phenomena occurring in alloys of this type at higher temperatures, it is essential to have data about the peculiarities and rates of the diffusion processes in them. Solution of the problem is fraught with serious theoretical difficulties. An attempt was made to examine this problem with no positive results. Pre-determined with certain values of diffusion coefficients in phases, and by applying the method of geometrical construction found an expression which connects the values of effective diffusion coefficient Def with the diffusion coefficients in each phase D1 and D2.